1. Field of the Invention
The present invention relates to creped paper. More-particularly, the invention relates to creped paper having desirable bulk, appearance, and softness characteristics, such that the paper is capable of being used for at least one of tissues, towels, and napkins. The invention also relates to a notched creping blade for use in a creped paper manufacturing process and a system including such a blade. The invention further relates to improved methods of manufacturing paper.
2. Description of Related Art
Paper is generally manufactured by dispersing cellulosic fiber in an aqueous medium and then removing most of the liquid. In particular, cellulosic fibers suspended in water are deposited on a moving foraminous support to form a nascent web. Water is removed from the nascent web, and the de-watered web is adhered to a heated cylindrical dryer (e.g., Yankee dryer). The web is then removed from the dryer.
Paper derives some of its strength from the mechanical interlocking of the cellulosic fibers in the web, but most of the strength is derived from hydrogen bonds that link the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness by consumers. One common method of increasing the perceived softness and cushion of bathroom tissue is to crepe the paper.
Creping is a process that typically includes fixing the cellulosic web to a cylindrical dryer (e.g., with an adhesive and release agent), and then scraping the web off of the dryer with a creping blade. Creping the paper advantageously breaks inter-fiber bonds, thereby increasing the perceived softness of the paper. However, creping with a conventional blade may not be sufficient to impart desired combinations of softness, bulk (i.e., thickness or caliper), and appearance to the paper. Therefore, creped paper for use as bathroom tissue generally requires additional processing after creping, particularly when produced using conventional wet pressing technology.
Paper produced using through air drying techniques normally have sufficient caliper, but may have an unattractive appearance. To overcome this shortcoming, an overall pattern can be imparted to the web during the forming and drying process by use of a patterned fabric having proprietary designs to enhance appearance. However, such patterned fabrics are not available to all producers. Moreover, through air dried tissues can be deficient in surface smoothness and softness, unless they are further processed using techniques such as calendering, embossing, and/or stratification of low coarseness fibers on the tissue""s outer layers.
Conventional tissues produced by wet pressing also generally require post-creping processes to impart softness and bulk. For example wet-pressed tissues are often calendered and/or embossed to bring softness and bulk parameters into acceptable ranges for premium quality products. Calendering, however, adversely affects caliper (i.e., thickness) and may raise the tensile modulus of the paper, which is inversely related to tissue softness. Embossing increases product caliper and can reduce the tensile modulus, but lowers strength and can decrease the surface softness of the paper. Accordingly, it can be appreciated that various combinations of calendering and/or embossing can have adverse effects on strength, appearance, surface smoothness, and thickness perception of the paper. In particular, there is a fundamental conflict between the use of calendering and the desire to increase the caliper of paper.
Conventional processes for creping paper using patterned or non-uniform creping blades are known. These processes, however, are suited for production of wadding, insulating papers, and other extremely coarse papers, but are not acceptable for production of premium quality bath tissue, facial tissue, and/or kitchen toweling.
Three references of interest are U.S. Pat. No. 3,507,745 to Fuerst, U.S. Pat. No. 3,163,575 to Nobbe, British Patent No. 456,032 to Pashley. Fuerst teaches the use of a highly beveled blade having square shouldered notches formed into the blade. The Fuerst blade is suitable for producing very high bulk for cushioning and insulation purposes, but is not generally suitable for premium quality towel and tissue products.
Nobbe discloses a doctor blade for differentially creping sheets from a drum to produce a product that is quite similar to the product described in the Fuerst patent. Nobbe teaches a flat blade having cut notches. The portions of the sheet that contact the notched portions of the blade will have a coarse crepe or no crepe, while the areas of the sheet that contact the unnotched blade portions will have a fine crepe.
The blade disclosed in Fuerst has a large bevel angle with portions of the creping edge being-flattened to produce a surface that results in fine crepe in the portions of the sheet that contact this surface. The portions of the sheet that contact the unmodified sections of the blade will have very coarse crepe, thus giving an appearance of having almost no crepe. Our experience suggests that neither the Nobbe nor the Fuerst blades are suitable for the manufacture of commercially acceptable premium quality tissue and towel products.
The Pashley reference teaches creping a sheet from a cylinder using a creping blade having an edge serrated in a sawtooth pattern. The teeth are disclosed as being about one-eighth (0.125) inch deep and having a frequency of about 8 per inch. The paper disclosed in Pashley is much coarser and more irregular than the crepe of a product made using conventional creping technology, and therefore not acceptable for use in premium tissue and towel products.
In light of the foregoing, there is a need in the art for an improved creped paper, creping blade, creping system, and method of producing paper.
Accordingly, the present invention is directed to creped paper, a creping blade, a creping system, and methods of producing paper that substantially obviate one or more of the limitations of the related art. To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes creped paper capable of being used for at least one of tissues, towels, and napkins. The creped paper includes a cellulosic web including crepe bars and undulations. Preferably, the cellulosic web includes recycled material. The crepe bars extend in a direction transverse to a machine direction and preferably have a spatial frequency of about 5 to about 150 crepe bars per inch. The undulations include ridges, furrows, crests, and sulcations extending longitudinally in the machine direction. The ridges and furrows are interspersed on the air side of the web, and the crests and sulcations are interspersed on a Yankee side of the web. The ridges preferably have a spatial frequency of about 5 to about 50 ridges per inch. A basis weight of the web is preferably about 7 to about 40 pounds per 3,000 square foot ream of the web. The undulations and crepe bars intersect to form a reticulum. The creped paper is preferably at least one of one-ply, multi-ply, embossed, calendered, wet-pressed, and through air dried.
In another aspect, the invention includes a creping blade for creping a cellulosic web from a rotatable cylinder in a creping process. The creping blade includes first and second side faces. The first side face is at least substantially opposite to the second side file. The blade also includes an upper surface adjacent to the first and second side faces. Preferably, the upper surface is not perpendicular to at least one of the first and second side faces. A plurality of notches are provided along the upper surface. Each of the notches has a bottom portion and an open end defined by at least a portion of the upper surface. The notches are configured to increase the caliper of the cellulosic web when the creping blade crepes the cellulosic web from an outer surface of the rotatable cylinder. In one embodiment, the notches are serrulate shaped; however, the notches could include a number of different shapes. The blade further includes an engagement surface adjacent to the upper surface and one of the first and second side faces. The engagement surface is dressed such that an angle between the engagement surface and the adjacent side face is approximately equal to a wear angle of the creping blade when the creping blade is positioned on an outer surface of the rotatable cylinder. A perpendicular distance between a lower portion of the engagement surface and an upper edge of the upper surface is at least as large as a perpendicular distance between the bottom portion of each of the notches and the upper edge. The engagement surface forms a substantially continuous line of contact with the outer surface of the rotatable cylinder when the creping blade is positioned on the outer surface, thereby obviating the need for substantial running in of the creping blade.
In a preferred embodiment, the perpendicular distance between the lower portion of the engagement surface and the upper edge of the upper surface is larger than the perpendicular distance between the bottom portion of each of the notches and the upper edge.
In a further aspect, the blade includes a plurality of protrusions that are adjacent to the notches and extend from the adjacent side face. Each of the protrusions preferably includes an engagement portion defining at least a part of the engagement surface. Preferably, the engagement portion of each protrusion extends from an edge of the bottom portion of a respective notch so that the edge intersects the adjacent side face. The plurality of protrusions are preferably spaced apart from one another. Preferably, the notches and protrusions are formed by displacing material from the creping blade.
In yet another aspect, the blade further includes rectilinear regions between the protrusions. The rectilinear regions are preferably formed when the engagement surface is dressed. Preferably, outer faces of the rectilinear regions form a portion of the engagement surface.
Preferably, the upper surface is beveled at an angle ranging from approximately 0xc2x0 to approximately 50xc2x0 with respect to a plane perpendicular to the adjacent side face. The frequency of the notches preferably ranges from approximately 5 per inch to approximately 50 per inch.
In still another aspect, the invention includes a system for creping a cellulosic web. The system includes a rotatable cylinder and at least one of the creping blades described above. The creping blade is positioned with respect to the cylinder so that the creping blade is capable of creping the cellulosic web from an outer surface of the cylinder when the web is on the outer surface and the cylinder is rotated.
In a further aspect, the invention includes a method of making paper, wherein a cellulosic web is creped from an outer surface of a rotatable cylinder with one of the creping blades described above. The cellulosic web preferably includes recycled material.
In yet another aspect, the invention includes a method of making paper, wherein cellulosic web is creped from an outer surface of a rotatable cylinder to produce one of the creped papers described above.
In another aspect, the invention includes a method of making paper, wherein one of the creping blades described above is placed in a mount adjacent to the rotatable cylinder.
Paper manufactured according to the present invention preferably is more capable of withstanding calendering without excessive degradation as compared to a conventional wet press tissue web. Accordingly, the paper making process is more forgiving and flexible than conventional processes. In particular, the present invention can be used to manufacture premium products including high softness tissues and towels having high strength and high bulk and absorbency, as well as paper having various combinations of bulk, strength and absorbency desirable for lower grade commercial products. For example, in commercial (i.e., away-from-home) toweling, it is generally considered important to have a relatively long length of toweling on a small diameter roll. In the past, this preferred feature has severely restricted the absorbency of commercial toweling products, because absorbency was adversely affected by the processing used to produce toweling having limited bulk (i.e., absorbency and bulk are directly proportional). Unlike conventional blades, the blade of the present invention preferably makes it possible to achieve high absorbency in a relatively non-bulky towel. Additionally, cellulosic web produced according to the present invention can be more heavily calendered than many conventional webs, while retaining bulk and absorbency. Thus the present invention preferably produces paper that is smoother and softer feeling, without unduly increasing the tensile modulus or unduly decreasing the caliper.
Paper made according to the present invention also saves on the cost of raw materials over conventional processes. In particular, the method of the present invention preferably can produce paper having a desirable degree of bulk at a low basis weight without an excessive sacrifice in strength, or it can preferably produce paper having a low percent crepe and a large caliper. Accordingly, it can be appreciated that the advantages of the present invention can be manipulated to produce novel products having many combinations of properties.
Furthermore, the method and creping blade of the present invention are at least comparable in runnability and forgivingness to conventional creping processes, and may be run on equipment adapted to use conventional creping blades. In particular, the creping blades of the present invention will fit into conventional holders and will operate at roughly equivalent holder angles. The life of the preferred blades is at least about the same as the life expected with conventional blades. At this time, preliminary results indicate that the life of preferred undulatory creping blades according to the present invention could possibly even be significantly greater than the life of a conventional blade, although to be able to claim this definitively would require a substantial amount of commercial operating data which are, of course, simply not available.
In contrast to conventional creped paper having creping bars generally running transversely, the tissue of the present invention has a biaxially undulatory surface, wherein the transversely extending crepe bars are intersected by longitudinally extending undulations imparted by the undulatory creping blade.
Besides the structural arrangements set forth above, the invention could include a number of other arrangements, such as those explained hereinafter. It is to be understood that both the foregoing description and the following description are exemplary, and are intended to provide further explanation of the invention as claimed.